function *fetchData(){
    // 性能
    let res1=yield(ajax(url,()=>{}))
    let res2=yield(ajax(url1,()=>{}))
    let res3=yield(ajax(url2,()=>{}))
}
let it=fetchData();// Generator
let result1=it.next()
let result2=it.next()
let result3=it.next()

// 缺点？